IEEE Microwave Magazine - May 2016 - 35

a side-lobe level of -30 dB. Note that TMAs also allow
a simpler architecture with respect to other beam
forming solutions, such as phased-arrays [18] or retrodirective arrays [19]: in fact, they do not need phase
shifters to create the proper phase condition at the
antenna ports.
Due to the superposition of the periodic switch
control sequences (at frequency f M) with the RF carrier
frequency f0, TMAs are able to radiate not only at the
fundamental carrier (f0, h = 0) but also at the sideband
harmonics (f0 + hfM, h ! 0) . This property has been
exploited in [14] for a smart two-step WPT procedure
via TMAs: first, a two-element time-modulated subarray is used for localization of tagged sensors to be
energized; then, the entire TMA provides the power to
the detected tags.
The TMA adopted in [14] for the smart WPT activity is the uniform 16-monopole planar array shown in
Figure 6: it operates at f 0 = 2.45 GHz, and is realized
on a Taconic RF60A substrate (f r = 6.15, thickness =
0.635 mm). The spacing between the elements is the
standard m/2 distance. As switching elements, microwave Schottky diodes (Skyworks SMS7630-079) are
used, driven by periodic sequences having a modulation frequency of f M = 25 kHz.

Partial Ground Plane

Nonlinear Switches

Figure 6. The layout of the uniform 16-monopole linear TMA
in [14].
harmonics, f0 ! fM , the shape of the difference (D) is
reproduced. By modifying the excitation duty cycle
(parameter d), it is also possible to scan the D pattern
in the way indicated in Figure 7(b): with the two inner
monopoles of the array shown in Figure 6, a scanning
region of about ±60° is achieved.
After a standard RFID reading operation to acquire
the tags' IDs, the sharpness of the negative peaks of the
steered D patterns allows high resolution in the localization phase; this fact can be exploited by means of
a suitable combination of the received signal-strength
indicators backscattered by the tags, using both the
R and D patterns, to build the maximum power ratio
(MPR) [21]:
MPR (i) = / dB
(i) - T dB
RSSI (i) .
RSSI

Localization of the Tags

The combination of the figure of merit (2) with the
scanning capability has proven its effectiveness in
indoor localization, with resolution up to few centimeters at 2.45 GHz [21]: the number of steps to discretize
the scanning window [21] is now realized by the same
number of control parameter d variations, providing
the associated switch's control sequences of the kind
reported in Figure 7(a).

In the first step of the WPT procedure, the TMA is
used to detect n tagged sensors. At this stage, the 14
peripheral switches are left open, and the sole dualinner-element subarray is operating. By piloting the
two switches in the way indicated in Figure 7(a) [20],
it is possible to obtain the fundamental radiation pattern at f 0 with the sum (R) shape, while at the first

Σ
∆ (δ = 16%)

Vdd

∆ (δ = 0)
∆ (δ = 24%)

∆ (δ = 8%)
∆ (δ = 32%)

0
τ
δ

0.5TM
Bias2(t )

TM

t

-10
-20
-30
-40
-50

τ

Vdd

-60
-90

δ

0.5TM

Normalized E (dB)

Bias1(t )

(2)

TM

-60

-30

0
θ (°)

60

90

t
f0 + fM

(a)

30

f0 - fM
(b)

Figure 7. (a) The control patterns of the two inner-array element switches, providing R and D patterns; and (b) the
corresponding simulated R and D radiation patterns for different d values [14].

May 2016

35



Table of Contents for the Digital Edition of IEEE Microwave Magazine - May 2016

IEEE Microwave Magazine - May 2016 - Cover1
IEEE Microwave Magazine - May 2016 - Cover2
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